1. Field of the Invention
The present invention relates to an imaging apparatus, sod, more particularly, to sheet picking system for an imaging apparatus.
2. Description of the Related Art
An imaging apparatus, such as a printer, scanner or copier, includes a sheet picking mechanism that is used to successively pick a single sheet of media, e.g., paper, from a media stack. With sheet picking mechanisms there is a critical normal force relationship between the pick roller of the sheet picking mechanism and the media stack. For example, too much normal force will result in feeds of multiple sheets at the same time, whereas too little normal force will result in failures to feed a sheet of media from the media stack. One type of sheet picking mechanism that attempts to overcome these problems includes a gear train, that is pivoted toward the media stack and rotates a drive roller with an increasing normal force being applied to the top sheet, of media until the top sheet of media is moved.
There is critical relationship between the buckling resistance of the sheet of media at the pick roller and the corresponding normal force at the media stack. Therefore, a simultaneous reaction happens between the buckling resistance and normal force at the media stock. In other words, a certain “cycle” exists during the picking process of the media wherein as the resistance in buckling of the media increases there is a corresponding increase in the normal force. This “cycle” can go on and on until either the sheet of media moves, the pick roller slips, or some part of the sheet picking mechanism tails. Also, it has been found that this increase in normal force for the corresponding buckling resistance increases from a media tray having a full media stack to a media tray that is empty. In other words, for the same buckling resistance the corresponding normal force is greater in an empty tray than in full tray causing a failure to pick, particularly in the case of relatively stiff media (e.g. cardstock, envelopes and index cards) at an almost empty tray level.
Because in picking heavier/stiller media the buckling resistance is large, causing a large normal force on the media stack which will increase the frictional resistance force between the top sheet and the next sheet, which in turn requires an increase in the needed torque to move the top media sheet. This action and reaction relationship between the drive force and the normal force may cause the system to fail to pick stiffer/thicker media.